Compressors , Fans & Blowers

INTRODUCTIONThe main use of the fans, compressors and blowers is the transportation of gasesThe main item of most processes is the compressor selectionThere are wide variety of compressors so it is crucial to define the operating conditions before selection

Major Factors During SelectionHead or Pressure RiseFlow RateTemperature LimitationsConsumption of PowerCost

The pressure rise which is the main difference between fans, compressors and blowers can be stated as follows:

P(psig) Fans 2 Blowers 2-10 Compressors >10

Compressors & Gas CompressionCategories and TypesCompression ProcessCompressor CharacteristicsKey Design ParametersCalculation MethodsSpecification Data SheetSelection GuidelinesControl SystemsTypical operating Problems

Compressor Application and Classification

Compressors are used in a variety of applications

Example: In natural gas plants, compressors are used to establish feed gas process pressures. Compressors also provide clean, dry air for instruments and control devices

In a refinery or chemical plant, compressors are used to compress gases such as light hydrocarbons, nitrogen, hydrogen, carbon dioxide, and chlorine

These gases are sent to headers, from which they are distributed to a variety of applications

Classification of Compressors

There are three basic designs for compressors : i) Dynamicii) Positive displacementiii) Thermal.

Dynamic compressors include centrifugal (radial flow) and axial (straight-line) flow compressors.

Dynamic compressors accelerate airflow by drawing air in axially and spinning it outward (centrifugal compressors) or in a straight line (axial flow compressors).

Positive displacement compressors include rotary and reciprocating compressors.

Positive displacement compressors compress gas into a smaller volume and discharge it at higher pressures.

Thermal compressors use ejectors to direct high-velocity gas or steam into the process stream, entraining the gas, then converting the velocity into pressure in a diffuser assembly.

Compressors Family Tree

Compressors & Gas CompressionCategories and Types

The principles of compression are: Gases and vapors are compressible. Compression decreases volume. Compression moves gas molecules close together. Compressed gases will resume their original shape when released. Compressed gases produce heat because of molecular friction. The smaller the volume, the higher the pressure. Force area = Pressure. Gas volume varies with temperature and pressure. Liquids and solids are not compressible (except under tremendous pressures).

Dynamic Compressors

@ Centrifugal Compressor

Gas enters a centrifugal compressor at the suction inlet and is accelerated radially by moving impellers. Centrifugal compressors have one moving element, the drive shaft and impeller. The impeller discharges into a circular, narrow chamber called the diffuser More sensitive to density and fluid characteristics Designed to operate at speeds in excess of 3000 rpm Can be single stage or multistage Single stage designed for high gas flow rates and low discharge pressure Multi stage designed for high gas flow rates and high discharge pressure can transferring wet product gas (rather than positive displacement)

The advantages of centrifugal compressors can be classified as;

They are more efficient than reciprocating onesThey provide high flowratesThey are compact, less site areaThey need lower maintainance requirementsThey are tolerant to liquid carry

Centrifugal Compressor

Compressors & Gas CompressionCentrifugal Compressor

Single-stage Centrifugal CompressorMulti-stage Centrifugal Compressor

Dynamic Compressors

@ Axial Flow Compressor

Normally used for jobs where highest flow and pressure required Request twice as many stages as centrifugal perform (8% to 10%) Primary application of axial compressors involves transfer of clean gas such as air Internal component are sensitive to corrosion, pitting and deposits More lighter, more efficient and smaller than centrifugal pumps Main purpose is in gas turbine applications

The advantages of axial compressors

They have higher efficiencyThey have higher capacity (flow rate)They are in smaller size

The disadvantages of axial compressors

They limited operating rangeThey are more subjected to corrosionThey are subjected to depositsThey have higher capital costsThey have lower heads

Axial Flow Compressor

Compressors & Gas CompressionAxial Compressor

Combined Axial and Radial Compressor ApplicationsThey have flow rates ranging from 50,000 to 690,000m3/hrThey have pressure ratio ranging from 5.8 to 12.5

POSITIVE DISPLACEMENT COMPRESSORSPositive Displacement CompressorsReciprocating CompressorsRotary Compressors

RECIPROCATING COMPRESSORSThey are the oldest type of compressorsThey have higher maintainance costs and lower capacity than dynamic compressorsThey are widely used in industryThey have cylinders which are equipped with suction and delivery valvesCompression cycle is composed of 3 cycles which are intake, compression and discharge

They intake gas by the help of cylindersThe pistons motion is reversed and the gas which taken in is compressedThe gas is expelled during the delivery stroke

In multistage reciprocating compressors;

The gas is compressed to an intermediate pressureThe other cylinders raise the pressure to the end pressureThere also exist intercoolers

Positive Displacement Compressors

@ Reciprocating piston Compressor

Work by tapping and compressing specific amount of gas between a piston and cylinder wall

The back and forth motion incorporated by a reciprocating compressor pull gas on the suction and discharge on the other

Spring loaded suction and discharge valves work automatically as piston moves up and down

Have a flexible pressure range and overall capacity, low power cost high efficiency rating

Reciprocating Piston Compressor

ROTARY COMPRESSORS

The two rotating components confine a volume of gasThe volume of the pocket decreases in rotation so pressure increases

The rotary compressors have high range of capacity and compression ratioThe rotary compressors are classified as; Lobed, Helical Screw, Sliding Vane

Positive Displacement Compressors

@ Rotary Compressor (Sliding Vane)

Use off center rotor with sliding vane to compress gases body (cast iron or steel), rotor and shaft ( high strength alloy steel), sliding vanes (asbestos-phenolic resin, metal) Does not use suction or discharge valve because it is designed to discharge against pressure

Positive Displacement Compressors

@ Rotary Compressor (Lobe)

Characterized by the two kidney bean shape impellers

Used to trap and transfer gases

Two impellers move on opposite direction during operation

Designed to have constant volume discharge pressure and constant speed drives

Can be used in wet and dry gas services

Also can be used as vacuum pumps

Positive Displacement Compressors

@ Rotary Compressor (Liquid Ring) Unusual compressor design (combines centrifugal action, with positive displacement and rotary action)

May be found in the following application :Hazardous gasesToxic gasesHot gases and vapor

Screw Compressor

HELICAL SCREW COMPRESSORSThere are mainly two screws which are called male and femaleThe gas is compressed between the lobes of the screw and move along the axis to an outlet port

These units can be; oil flooded and dryThe contamination of oil is prevented by dry compressorsOil flooded units are used in refrigeration systems and plant air service

FANS & BLOWERS

Blower and Fans

Simple devices typically classified as compressors

two basic design (axial flow and centrifugal flow)

mostly are single stage devices.- centrifugal blower (low pressure air systems, refrigeration unit or laboratory hoods) - fan (direct airflow into or out of ind. equipment such as cooling tower, boilers or HVAC system)

centrifugal fan to move gases over a wide range of conditions

FANSThey are the air displacement systems moving air continuously to moderate pressuresDue to little change in pressure of air in fans, air is considered to be incompressibleThey can have pressure rise up to 2 psig.

The characteristics of fans can be classified as;The volumetric flowrate of the gas displaced by the fan is directly proportional with the fan speedThe static pressure varies with the square of the fan speedThe power consumed varies with the cube of the fan speed

FANSAxialCentrifugalTubeVaneRadial BladeForwardCurvedBackward CurvedAir Foil

AXIAL FANS Gas moves parallel to the axis of rotation

There are two types of axial fans;Tube axial fansVane axial fans

Tube axial fans are used for wide range of volumes at medium pressure

In vane axial fans there is air guide vane on the discharge side and the air flow pattern is a straight line hence improvement in efficiency and reducement in turbulance is observed

CENTRIFUGAL FANSGas stream moves perpendicular to the axis of rotationThey are classified as; radial blade, forward curved, backward curved and air foil

RADIAL BLADE CENTRIFUGAL FANSThey are used for pneumatic transportation and exhausting process gas in high resistance systemsWith relatively low capacity, they can achive high static pressureThey can develop high pressures with high speedsBlades clean themselvesThey are not used for ventilating purposes

FORWARD CURVED CENTRIFUGAL FANSThey discharge higher volume of air at slower fan speeds

They operate with a moderate amount of noise

They require little space

They are used for clean gases

BACKWARD CURVED CENTRIFUGAL FANSThey develop much of their energy directly as pressure

They develop less velocity heads by operating at medium speeds

Small variations in system volume result in small variations in air pressure

AIR FOIL CENTRIFUGAL FANSThey are backward curved centrifugal fans with an air foil cross section

They can operate more silently since air forms no turbulance while flowing through the wheels

BLOWERSBlowers are used for supplying low pressure air up to between 2-10 psig.They consist of two parallel shaft rotorsThey may have 2 4 lobesThe rotating shaft in the constitution of the blower traps some gasThe compression of the gas in the blower is negligible

Blowers

They are used for;

Pneumatic transportation of particulate material

Water and waste treatment

Providing moderate vacuum

Compressors & Gas CompressionRanges of Application

Compressors & Gas CompressionCompression ProcessGas compression is a thermodynamic process where change takes place in the physical state of the gas

Compression adds energy to the gas resulting in pressure-volume changes defined by ideal gas laws

Compression take place under conditions defined:Adiabatic:no heat added or removed from systemsIsothermal:constant temperature in systemPolytropic:heat added or removed from system

Compression of real gases in actual compressors deviate from conformance with ideality, usually significantly, affecting compressor design.

Compressors & Gas CompressionCompressor CharacteristicsCapacity/Head

Performance

Terminology

Compressors & Gas CompressionReciprocating CompressorPerformance Diagram

TerminologyPiston DisplacementClearance VolumeVolumetric EfficiencyPressure RatioRod Loading

Compressors & Gas CompressionReciprocating Compressor

Compressors & Gas CompressionReciprocating Compressor

Compressors & Gas CompressionCentrifugal CompressorPerformance Curves

TerminologyOperating PointSurge PointStonewallStabilityTurndown

Compressors & Gas CompressionCentrifugal Compressor

Compressors & Gas CompressionCentrifugal Compressor

Compressors & Gas CompressionCentrifugal Compressor Performance

Compressors & Gas CompressionCentrifugal CompressorKey Design ParametersCapacityGas PropertiesPressure HeadPowerEfficiencyMulti-Stages

Compressors & Gas CompressionCentrifugal CompressorKey Design Parameters

Flow RatesNormalMaximumMinimum

Design CapacityCapacity

Compressors & Gas CompressionCentrifugal CompressorKey Design ParametersCompositionContaminantsMolecular Weight MWSpecific Heat Ratio Cp/CvCompressibilityGas Properties

Compressors & Gas CompressionCentrifugal Compressor10C38C66C93C121C

Compressors & Gas CompressionCentrifugal Compressor

Compressors & Gas CompressionCentrifugal Compressor

Compressors & Gas CompressionCentrifugal Compressor100F = 560R: 560/549 = 1.02100F = 311K, 549R = 305K: 311/305 = 1.02PV = ZmRT/MWP=100psia = 6.89 bar a T=100F = 37.8C = 310.9K = m/V = P(MW)/(ZRT)= 6.89E5x34.27/(0.946x8314x310.9)= 9.7kg/m3= 0.61lb/ft3

Compressors & Gas CompressionCentrifugal Compressor0.9730.0771.02

Compressors & Gas CompressionCentrifugal Compressor0.88

Compressors & Gas CompressionCentrifugal CompressorKey Design ParametersAvailable vs. Required HeadAvailable Head is Compressor RelatedH(Available) = CV2/gC = Pressure Coefficient (0.55)Required head is System-RelatedHeadH(Required)

Compressors & Gas CompressionCentrifugal CompressorFor centrifugal compressors the following method is normally used:

First, the required head is calculated. Either the polytropic or adiabatic efficiency is used with the companion head.Horsepower Calculation

Compressors & Gas CompressionCentrifugal CompressorHorsepower CalculationWhere:Z=Average compressibility factor: using 1 will yield conservative resultsR= 1544/(mol weight)T1= Suction Temperature, RP1, P2= Suction, discharge pressures, psiaK= Adiabatic exponent, (N-1)/N = (K-1)/(KEp)Ep= Polytropic EfficiencyEA= Adiabatic Efficiency

Compressors & Gas CompressionCentrifugal CompressorHorsepower CalculationThe polytropic and adiabatic efficiencies are related as follows:From Polytropic Head:

HP = WHpoly/(Ep 33000)From Adiabatic Head:

HP = WHAD/(EA 33000)Where:

HP = Gas Horse PowerBHP = Brake HorsepowerW = Flow, Lb/minBHP = HP/Em

Compressors & Gas CompressionEfficiencyHydraulic EfficiencyAdiabaticPolytropic

Volumetric EfficiencyReciprocating

Mechanical EfficiencyDrivers

Compressors & Gas CompressionCentrifugal CompressorApproximate polytropic efficiencies for centrifugal and axial compressors

Compressors & Gas CompressionTemperature RiseTemperature ratio across a compression stage is:

T2/T1 = (P2/P1)(K-1)/KAdiabatic

T2/T1 = (P2/P1)(N-1)/NPolytropic

Where:

K = Adiabatic exponent, Cp/CvN= Polytropic exponent, (N-1)/N = (K-1)/KEpP1, P2 = Suction, discharge pressures, psiaT1, T2 = Suction, discharge temperatures, REp = Polytropic efficiency, fraction

Compressors & Gas CompressionTemperature RiseThe usual centrifugal compressor is uncooled internally and follows a polytropic path.

Temperature must often be limited to:Protect against polymerization as in olefin or butadiene plantsAt T > 230-260C, the approximate mechanical limit, problems of sealing and casing growth start to occur.

High temperature requires a special and more costly machine. Most multistage applications are designed to stay below 250-300C

Compressors & Gas CompressionTemperature RiseIntercooling can be used to hold desired temperatures for high overall compression ratio applications.This can be done between stages in a single compressor frame or between series frames.

Sometimes economics rather than a temperature limit dictate intercooling.

Sometimes for high compression ratios, the job cannot be done in one frame. Usually a frame will not contain more than 8 stages (wheels). For many applications the compression ratio across a frame is about 2.5 4.0

The maximum head that one stage can handle depends on gas properties and inlet temperature. Usually this is about 2000 to 3400m for a single stage.

Compressors & Gas CompressionSurge ControlsA centrifugal compressor surges at certain conditions of low flow.

Surge control help the machine to avoid surge by increasing flow.For an air compressor, a simple spill to atmosphere is sufficient.For a hydrocarbon compressor, recirculation from discharge to suction is used.

Compressors & Gas CompressionSurge ControlsThere are many types of surge controls.

Avoid the low-budget systems with a narrow effective range, especially for large compressors.

Good systems include the flow/P type.

The correct flow to use is the compressor suction. However, a flow element in the suction can rob excessive horsepower. Therefore, sometimes the discharge flow is measured and the suction flow calculated within the controller by using pressure measurements. The compressor intake nozzle is also sometimes calibrated and used as a flow element.

Compressors & Gas CompressionCompressor Calculation MethodDefine gas properties: MW, Cp/Cv, Z 1Define inlet conditions: Temp & Press.Calculate gas flow rate: Normal and Design 1Establish total discharge pressure.Calculate compression ratio and number of stagesDefine selection & polytropic efficiency

1. At inlet conditions

Compressors & Gas CompressionCompressor Calculation Method contdCalculate heat capacity factor MCalculate required polytropic headCalculate hydraulic gas horsepowerCalculate discharge temperatureCalculate total brake horsepowerEstimate inter-stage cooling requirement

Compressors & Gas CompressionCompressor Calculation Example 1:Calculate compressor required to handle a process gas at the following operating conditions: Inlet press and temp at 20 psia and 40F. Discharge pressure of 100 psia. Gas rate 2378 lb.mol/hr of the following composition and calculated properties:

Mol%Mol/hMol.WtCpTcPcEthane24830.10.6011.960.245501170814Propane95225944.141.916.5515.70666633617587Butane37158.11.7422.500.687662355117Total100237844.2416.62667618

Compressors & Gas CompressionCompressor Calculation Example 1: contdInlet flow:

Weight flow = 2378 x 44.24/60 = 1753 lb/min

Pr = 20/618 = 0.0324, Tr = (40+460)/667 = 0.75Compressibility factor Z = 0.97 (from generalized Z chart)

Density= (MW x P1)/(10.73 x T1 x Z)= (44.24 x 20)/(10.73 x (40 + 460) x 0.97)= 0.17 lb/cu.ft

Inlet volume = 1753/0.17 = 10 310 cu.ft/minCalculation:

Compressors & Gas CompressionCompressor Calculation Example 1: contdHeat Capacity Factor

k = Cp/Cv = Cp/(Cp 1.99) = 16.62/(16.62 1.99) = 1.137

M = (k-1)/(kEp)

Assume Ep = 77%:M = (1.137 1)/(1.137 x 0.77) = 0.156Calculation:

Compressors & Gas CompressionCompressor Calculation Example 1: contdPolytropic Head, Hp

Calculation:= 0.97 x (1545/44.24) x (40 + 460)/0.156 x [(100/20)0.156 -1]= 30 988 ft

Compressors & Gas CompressionCompressor Calculation Example 1: contdDischarge Temperature, T2

T2= T1(P2/P1)M= 500(5)0.156= 643R= 183F

Gas Horsepower (GHP) & Brake Horespower (BHP)

GHP= W . Hpoly/(33000Ep)= 1753 x 30988/(33000 x 0.77)= 2140

BHP= 2140/0.98 = 2180 (Assume Mechanical Eff. = 98%)Calculation:

Compressors & Gas CompressionExampleCalculate the Brake Horsepower for the following Compressor:

Compressors & Gas CompressionExampleCalculate the Brake Horsepower for the following Compressor:Calculate Gas Mixture Properties

Composition:H2 = 65.6/(65.6+21.4) = 75.4 vol%N2 = 100 75.4 = 24.6 vol%CompositionMole%Mole WtMWmass%CpMWHydrogen75.421.5118.014.32.57Nitrogen24.6286.89821.040.85Total Gas Mix100.08.4011.043.42Use Z = 1 for conservative results

Compressors & Gas CompressionExampleCalculate the Brake Horsepower for Compressor: ContdLets look at the first stage:

First calculate Polytropic Head:T2/T1= (P2/P1)(N-1)/Nln(T2/T1)= (N-1)/N ln(P2/P1)(N-1)/N= ln(T2/T1)/ln(P2/P1)= ln(372/295)/ln(4400/2518)= 0.416

Hpoly= 1 x (8.314/8.4) x 295 x ((4400/2518)0.416 -1) 0.416= 183.4 kJ/kgT1 = 22C = 295KT2 = 99C = 372KP1 = 2418 kPag = 2518 kPa a P2 = 4300 kPag = 4400 kPa a

Compressors & Gas CompressionExampleCalculate the Brake Horsepower for Compressor: ContdFirst stage:(N-1)/N= (K-1)/(KEp) Ep= (1.4 -1)/(1.4 x 0.416)= 0.69

W = (107 000/22.414) x 8.4 = 40100kg/h = 11.14 kg/sCp/Cv= Cp/(Cp-R)= 3.42/(3.42-8.314/8.4)= 1.4

Compressors & Gas CompressionExampleCalculate the Brake Horsepower for Compressor: ContdFirst stage:Gas Horsepower= W . Hpoly/Ep= (11.14 x 183.4)/0.69= 2960 kJ/s= 3.0 MW

Similar for stage 2, 3 and Recycle:GHP(stage 2) = 2.9MWGHP(stage 3) = 3.3 MWGHP(recycle stage) = 1.0 MWTotal GHP = 3.0 + 2.9 + 3.3 + 1.0 = 10.2 MW

A good assumption for Mechanical Efficiency = 95%

BHP = 10.2/0.95 = 10.6 MW

Compressors & Gas Compression

Compressors & Gas Compression

Supporting Equipment in a Compressor System

Supporting Equipment in a Compressor System

Intercooler and after cooler heat exchanger - compression of gases create heat in compressor- control high temperature- intercooler lower the temperature as gas is discharge out of first stage of compressor- as the gas is compressed (create more heat), discharge into after cooler before go to receiverSafety valve- used to relieve excess pressure that could damage operating equipment- sized to handle specific flow rates

Supporting Equipment in a Compressor System (cont.)

Silencers- most compressor exceed OSHA standards noise pollution- muffle some of the damaging noise produced by compressor- should be mounted on the inlet and outlet of a compressor Demister- designed to remove liquid droplets from gas- function as a cyclone- heavier component fall to the bottom of the demister and removed- clean gas escapes out the discharge line on the top of the demister Dryer - for dry air service, discharge of a compressor is run through a dryer- filled with moisture adsorbing chemicals called desiccant dryer(alumina, mol sieves and silica gel)- operation uses parallel or series dryer

Compressor system

Start up and shutdown a dynamic compressor

Start up and shutdown a positive displacement compressor

Troubleshooting a centrifugal compressor

Troubleshooting a reciprocating compressor

Compressor Symbols

CONCLUSIONSThe fans have wide range of flowrateThe material selection is important during manufacturing fansThe blowers have low power and pressure applicationsThe blower is less efficient method of compression

CONCLUSIONS

The centrifugals are tolerant to liquid carryThe liquid with the gas can cause erosion and severe damage in centrifugalsIn axial equipment, high compression efficiency is observedThe axial equipment is applied for high flow andlow discharge pressures

CONCLUSIONSThe reciprocating systems are applicable for low flow rate of high pressure ratioThe oil contamination is important in reciprocating systemsThe reciprocating systems have higher maintainance costThe reciprocating systems are not suited to dirty gassesThe process gases that are taken in should be clean and dry in axial equipmentsThe reciprocating systems are not tolerate liquid droplets in the suction flow

CONCLUSIONSThe screw compressor have higher initial cost than reciprocating compressors for the same dutyThe sliding vane compressors have low pressure applicationsThe sliding vane compressors operate at low speedsThe noise level of the sliding vane compressors is low

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